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Both alkyne (C≡C) and nitrile (C≡N) functional groups contain triple bonds and show stretching absorptions around the wavenumber range of 2100 to 2300 cm−1 in the diagnostic region of the IR spectra.
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Introduction:
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Infrared spectroscopy, also known as vibrational spectroscopy, is mainly used to determine the types of bonds and functional groups in molecules. In aldehydes and ketones, the carbonyl (C=O) bond shows an absorption around 1710 cm-1. The C=O bond vibration of an aldehyde occurs at lower frequencies than that of a ketone. In addition to the C=O absorption in an aldehyde, the aldehydic C–H bond also gives two peaks in the 2700–2800 cm-1 range. This absorption, coupled with the...
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¹H NMR: Long-Range Coupling01:27

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The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
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A Near-Threshold Shape Resonance in the Valence-Shell Photoabsorption of Linear Alkynes.

U Jacovella1, D M P Holland2, S Boyé-Péronne3

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Internal alkynes exhibit enhanced photoabsorption cross sections due to a specific shape resonance. This phenomenon, linked to the highest occupied molecular orbital and l=4 continuum, is absent in terminal alkynes.

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Area of Science:

  • Physical Chemistry
  • Quantum Chemistry
  • Molecular Spectroscopy

Background:

  • Linear alkynes with internal triple bonds display unique photoabsorption characteristics.
  • Previous studies on 2-butyne indicated resonance features above the ionization threshold.
  • Comparison between internal and terminal alkynes reveals significant differences in photoabsorption cross sections.

Purpose of the Study:

  • To investigate the origin of enhanced photoabsorption cross sections in internal alkynes.
  • To assign the observed spectral features to specific electronic transitions and resonance states.
  • To theoretically confirm the role of shape resonances and partial waves in photoabsorption spectra.

Main Methods:

  • Measurement of room-temperature photoabsorption spectra for various linear internal alkynes.
  • Theoretical calculations of absorption spectra above the ionization threshold for 2- and 3-alkynes.
  • Analysis of electronic structure, including highest occupied molecular orbital (HOMO) and virtual orbitals.

Main Results:

  • Internal alkynes show similar resonances just above the ionization threshold, enhancing photoabsorption cross sections.
  • These features are assigned to excitation from the HOMO to a shape resonance with g (l=4) character and π symmetry.
  • Theoretical calculations confirm the presence of an l=4 shape resonance in internal alkynes, absent in terminal alkynes.

Conclusions:

  • The enhanced photoabsorption in internal alkynes is attributed to a generic shape resonance involving l=4 partial waves.
  • This behavior stems from the similar electronic structure of HOMOs and virtual orbitals in internal alkynes.
  • The findings provide a comprehensive understanding of photoabsorption processes in linear alkynes and related species.